Study 22001, "A Double-Blind, Placebo-Controlled, Flexible-Dose Evaluation of the Efficacy, Safety, and Tolerability of STX209 in the Treatment of Irritability in Subjects with Fragile X Syndrome" currently is evaluating the efficacy of STX209 (R-baclofen) for management of typical problem behaviors, such as irritability and aggression, in subjects with FXS. This study (22002) will enter subjects who complete Study 22001 into a long-term, open-label study.The open-label extension protocol will provide necessary data on the long-term safety and tolerability of STX209 among subjects with FXS who receive treatment under conditions more closely reflective of their general medical care.